Coral organisms, called polyps, can live on their own, but are primarily associated with the spectacularly diverse limestone communities, or reefs, they construct.

Coral polyps are tiny, soft-bodied organisms related to sea anemones and jellyfish. At their base is a hard, protective limestone skeleton called a calicle, which forms the structure of coral reefs. Reefs begin when a polyp attaches itself to a rock on the sea floor, then divides, or buds, into thousands of clones. The polyp calicles connect to one another, creating a colony that acts as a single organism. As colonies grow over hundreds and thousands of years, they join with other colonies and become reefs. Some of the coral reefs on the planet today began growing over 50 million years ago.

Coral polyps are actually translucent animals. Reefs get their wild hues from the billions of colorful zooxanthellae (ZOH-oh-ZAN-thell-ee) algae they host. When stressed by such things as temperature change or pollution, corals will evict their boarders, causing coral bleaching that can kill the colony if the stress is not mitigated.

Corals live in tropical waters throughout the world, generally close to the surface where the sun's rays can reach the algae. While corals get most of their nutrients from the byproducts of the algae's photosynthesis, they also have barbed, venomous tentacles they can stick out, usually at night, to grab zooplankton and even small fish.

Coral reefs teem with life, covering less than one percent of the ocean floor, but supporting about 25 percent of all marine creatures. However, threats to their existence abound, and scientists estimate that human factors—such as pollution, global warming, and sedimentation—could kill 30 percent of the existing reefs in the next 30 years.